36 yo female with 35 lb gain over 24 mo, no change to diet or activity. hx of ibs-d, recently increased in severity.potential helminth in recent stool, wondering if this is, in fact, a parasite and if could be the cause of weight gain and gi upset?

I wouldn't base any weight gain on a helminth (worm-parasite) infestation if it is present. If anything it could cause some GI Upset and weight loss. I would have a thorough evaluation by your doc followed by a consultation with a clinical dietitian. Many times the estimate we have of what we consume differs from the reality shown by analysis of an expert.

If you have not travelled to a poor country, there is virtually zero chance of helminthic infestation. Metabolism slows down with age and you need to reduce your food intake. It may sound drastic, but aim to reduce your intake by half. Wish you good health!